Do not overwrite event mask in WindowThumbnailItem

The WindowThumbnail needs XCB_EVENT_MASK_STRUCTURE_NOTIFY on the window.
For this it needs to change the window attributes. But it needs to keep
the existing event mask to not break other code. Also it cannot remove
the event mask again, as there is no chance to know whether another
component requires STRUCTURE_NOTIFY.

This fixes the not updating items in the tasks applet as the event mask
needed for KWindowSystem got overwritten.


REVIEW: 118137